BEM

Question 1

What does BEM mean?

Answer 1

Blade Element Momentum Theory

Question 2

When is BEM being used?

Answer 2

• wind turbine is operating outside of optimum performance points
• arbitrary operation states
• betz or schmitz can not be used

Question 3

What is calculated with BEM?

Answer 3

• calculation of inflow conditions, aerodynamic forces and power output
• used to calculate the rotor performance for all arbitrary operational conditions

Question 4

On what principle is BEM based on?

Answer 4

• that forces and moments from momentum theory and blade element theory are in equilibrium

Question 5

What is the thrust force?

Answer 5

Force perpendicular to the rotor plane exerted on the flow resulting in deceleration of the wind velocity

Question 6

How can Momentum theory be summerized?

Answer 6

• look at the flow and calculate the force that is responsible for the deceleration of the actual wind velocity, so the thrust force
  and
  we calculate the torque that is responsible for the creation of the wake rotation

Question 7

What is the fundamental difference between the momentum theory and the blade element theory?

Answer 7

Momentum theory: - observed the flow and calculated the torque and thrust force that is acting on the flow
Blade element theory: calculate the torque and thrust force that is acting on the rotorof the wind turbine

Question 8

What are the assumptions in Blade element theory?

Answer 8

• spatial discretization: for each blade element, consider the chord length to be constant
• account for the actuarter disc theory
• do not consider any aerodynamical interactions between different blade elements
• blade elements are uniformly loaded
• forces acting on blade elements are solely determined by the lift and drag coefficients

Question 9

What is local solidity?

Answer 9

• the portion of blocked surface in the annulus ring
• the sum of the chord length of all blades in the annulus ring devided by the overall circumference of the annulus ring
• ratio of blocked surface

Question 10

How can Blade Element theory be summerized?

Answer 10

• calculate the thrust force and torque on blade element level via the inflow triangle and via the lift and drag coefficients (and trigonometry)
• thrust force and torwue according to the blade element theory are the forces that the flow exerts on the rotor of the wind turbine

Question 11

Why do we need an iteration process to calculate a and a’?

Answer 11

• the inflow triangle is not known a priori
• the inflow triangle depends on a and a’ but a and a’ also depend on the inflow angle which is one part of the inflow triangle
• thats why you can not solve the equations directly

Question 12

Why do we need Glauert correction?

Answer 12

• if the axial induction factor is large then there are a lot of eddies and turbulences in the wake flow
• results in an energy transfer at the boundaries of the stream tube
• particles crossing the boundaries (that violates assumptions of stream tube)

Question 13

How can BEM be summerized?

Answer 13

• iterative procedure to calculate the inflow with whcih the aerodynamic forces and thus the power output can be calculated
• combination of MT and BET